Expanded molded articles comprising a polystyrene-based resin have been frequently used as packaging materials and thermal insulating materials because such expanded molded articles have excellent shock-absorbing and thermal insulating properties and are readily formable. The expanded molded articles are, however, insufficient in impact resistance and in plasticity and thus become cracked or chipped easily. Therefore, the expanded molded articles are not suited for packaging some items such as precision apparatuses.
On the other hand, expanded molded articles comprising a polyolefin-based resin are excellent in impact resistance and in plasticity but require large-scale equipment at a time of molding these expanded molded articles. Moreover, because of the properties of the resin, the polyolefin-based resin needs to be transported in the form of pre-expanded particles from a raw material maker to a molding and processing maker. Since the pre-expanded particles that are bulky need to be transported, some problems arise such as high production costs.
Because of the above reasons, various polystyrene-based composite resin particles which have the merits of the above-described two resins and expanded molded articles using these resin particles have been suggested.
For example, WO 2007/138916 (Patent Document 1) discloses a method for producing expandable polyethylene-based resin particles comprising polymerizing core particles containing an ethylene-vinyl acetate copolymer and a linear low-density polyethylene with a styrene-based monomer or a monomer mixture containing a styrene-based monomer and immersing a blowing agent therein.
An object of the technique is to provide expandable polyethylene-based resin particles and an expanded molded article which have excellent expansion moldability over a prolonged period of time and maintain impact resistance, bending deflection and restoration from repetitive stress/strain, which are characteristic of expanded olefin-based resin molded articles. Patent Document 1 discloses that the object is achieved by polymerizing a styrene-based monomer in the presence of core particles containing a specific component and immersing a blowing agent therein.
WO 2006/027944 (Patent Document 2) discloses that a modified polyethylene-based resin expanded molded article which is obtained by expansion molding of a non-cross-linked linear low-density polyethylene-based resin obtained by polymerization using a conventional Ziegler-Natta catalyst is insufficient for automotive members such as shock-absorbing materials, interior trims and bumpers for which impact resistance is required and cannot fulfil further high impact resistance. Patent Document 2 further points out that in a method wherein such a polyethylene-based resin without an inorganic nucleating agent is used, resulting modified resin particles may have a styrene-based resin dispersed in the form of particles in the vicinity of the surface of the particles while the styrene-based resin is hardly dispersed in the form of particles at the central part of the particles, resulting in a continuous phase, and thus cannot fulfil high impact resistance. On the basis of the above, Patent Document 2 discloses a technique of which object is to provide styrene modified linear low-density polyethylene-based resin particles which provide an extremely excellent expanded molded article having an extremely excellent strength.
According to the technique, a styrene-based monomer is added to linear low-density polyethylene-based resin particles obtained by polymerization using a metallocene compound as a catalyst and then the styrene-based monomer is polymerized at a temperature which is 10 to 35° C. higher than the crystallization peak temperature of the low-density polyethylene-based resin. Patent Document 2 discloses that, because of this, the styrene-based resin can be dispersed in the form of particles not only in the vicinity of the surface of the particles but also in the vicinity of the central part thereof and styrene modified linear low-density polyethylene-based resin particles can be obtained which can provide an expanded molded article sufficiently exhibiting impact resistance of the ethylene-based resin and rigidity of the styrene-based resin.
Further, WO 2007/099833 (Patent Document 3) discloses a technique an object of which is to provide a styrene modified polypropylene-based resin expanded molded article which has excellent mechanical properties and chemical resistance and remedies the defects of both the polystyrene-based resin expanded molded article and the polypropylene-based resin expanded molded article.